In antenna systems, such as satellite antenna systems used, for example, in a global positioning system (GPS) or in a communications system, multiple closely spaced or overlapping pencil beams are produced to cover a particular country or a geographic area. For design purposes, the covered area may be defined as a polygon with edges corresponding to a geographical or political boundary. The coverage polygon should be fitted with a regular pattern of circular or slightly elliptical beams using a hexagonal or honeycomb lattice as the underlying basis. For example, FIG. 1 illustrates 10 beams covering the continental United States (CONUS).
The coverage area may comprise a group of neighboring countries, with each beam covering a different language or cultural region. In this situation, each beam must be centered optimally over its assigned region, with the fitting process unavoidably requiring irregular beam spacing, size, and even shape. For example, FIG. 2 shows a system of irregularly spaced beams placed over western and central Europe.
Therefore, there is a need to maximize the antenna gain (directivity) over the coverage area, to ensure that the minimum gain exceeds the specification required by the communications link budget. Hence, very closely spaced or overlapping beams should be formed. The amplitude vs. angle shape in current design practice has the approximate form of sin(x)/x or J1(x)/x, and sometimes is referred to as paraboloidal (when describing the pattern function within a few dB from peak). When a hexagonal lattice of circular beams with a paraboloidal [sin(x)/x] shape of radiation pattern is used, the conventional spacing between adjacent beams is approximately equal to the 3 dB beamwidth of the antenna. The minimum gain within the coverage area occurs at the triple crossover point of any three neighboring beams, and is about 4 dB below peak. When the beams are irregularly spaced, the beam spacing may be smaller or larger than the 3 dB beamwidth, and the crossover level may vary from 2 to 6 dB below peak if a paraboloidal [sin(x)/x] beam shape is used.
Optimum beam placement and compliant minimum gain are the primary goals for designing such antenna systems. However, other performance requirements and design constraints need to be considered, and may be equally important. For example, polarization, bandwidth, frequency reuse schemes, cross-polar isolation, co-polar (sidelobe) isolation for a frequency-reuse system of beams, maximum gain variation within a beam area, pointing error, antenna size, mass, and cost.
Traditionally, antennas having paraboloidal main reflectors are used for producing multiple beams. However, as illustrated in FIG. 3, when an antenna designer attempts to produce a system of closely spaced beams from a single antenna having a paraboloidal (P) main reflector, with beam spacing S and size B approximately equal to the 3 dB beamwidth of the antenna and with one feed per beam, the design problem becomes highly complex. The problem is that small beam spacing necessitates small feed size. Due to laws of optical ray tracing, the antenna geometry maps directly the spacing between beams into spacing between feeds. Feed elements with small aperture diameters produce feed radiation patterns with very large beamwidths that exceed the angle subtended by the reflector rim as viewed from the feed. As a result, a large fraction of power radiated by the feed flows outside of the reflector. This power is called the spillover loss. The fraction of feed power intercepted or captured by the reflector is called the spillover efficiency. With the feed diameter constrained to a small value by the beam layout, the spillover efficiency is typically less than 50%, i.e. the spillover loss is quite large, e.g. in the range of 3 to 5 dB. Except under special circumstances, an antenna design with such a poor efficiency is not acceptable.
It is possible to increase the feed size by selecting antenna geometry with a larger F/D ratio, where F is the focal length of the reflector, and D is the aperture diameter of the reflector. A typical value of this ratio for practical designs is F/D=1.0. Since aperture diameter D is fixed by the specified beamwidth of the pencil beams, a larger F/D ratio is achieved by increasing the focal length F. A larger F/D ratio increases the proportionality constant relating the feed spacing to the beam spacing. A feed with a larger diameter produces a feed pattern with a narrower beamwidth. However, when the focal length is increased, the angle subtended by the reflector rim becomes smaller (as viewed from the feed, which is now at a greater distance). The feed pattern is more focused, but the angular area intercepted by the reflector becomes smaller. Therefore, the spillover efficiency remains just as bad as for the traditional F/D ratio, or improves by an insignificant amount.
Conversely, it is possible to increase the angle subtended by the reflector by decreasing the F/D ratio, i.e. selecting antenna geometry with a shorter focal length F. However, a smaller F/D decreases the proportionality constant relating the feed spacing to the beam spacing. Hence, smaller feeds with a larger beamwidth should be used. The angular area intercepted by the reflector becomes larger, but because the feed pattern is less focused, the spillover efficiency again has not improved. Moreover, other design constraints associated with small feed size need to be carefully considered, for example propagation cutoff in feed waveguide, mutual coupling, and input impedance matching.
Accordingly, since the beams are closely spaced or effectively overlap, the feed apertures in the feed plane (the focal plane images of the beam areas) should also overlap. But this is impossible since two or three feeds cannot occupy the same area in the feed plane.
The above discussion shows that a conventional design of a multi-beam antenna using a single aperture and one feed per beam is handicapped by an extremely poor efficiency, with the spillover loss exceeding 3 dB. Depending on the antenna geometry, feed size, and feed type, other losses may also become significant: loss due to mutual coupling and loss due to power reflected from the feed input (input match).
To overcome the design difficulties described above, the conventional methodology uses an antenna system with multiple apertures. As illustrated in FIG. 4, in the antenna system with multiple apertures, large feeds may be distributed among three reflector antennas. In each antenna, the main reflector has a paraboloidal surface with a diameter defined by the beamwidth of the pencil beam with an F/D ratio approximately equal to 1.0. As alternate feeds are distributed among three separate feed planes belonging to different independent reflector antennas, they do not interfere. For a hexagonal beam/feed lattice, with a three-aperture solution, the feed diameter can be 1.73 times larger than the feed diameter for the one-aperture solution; for a four-aperture configuration, the feed diameter is two times larger. Therefore, the feed pattern beamwidth is either 1.73 or 2.0 times narrower, the feed radiation is much better focused on the reflector and the spillover efficiency improves, with the spillover loss dropping to around 0.5–0.6 dB.
An example of a simple three-aperture design producing a cluster of seven beams on a hexagonal lattice is illustrated in FIG. 5 that shows a perspective view of the three antennas and their feeds mounted on a spacecraft body. Each antenna has a single offset paraboloidal reflector, and one or multiple feeds that radiate electromagnetic energy illuminating the respective reflector. For example, antenna 1 may comprise a reflector 12 illuminated by energy radiated by a single feed 14, antenna 2 may comprise a reflector 16 illuminated by energy radiated by a feed cluster 18 composed of three feeds, and antenna 3 may have a reflector 20 and a feed cluster 22 composed of three feeds. Also, FIG. 5 shows various elements of spacecraft environment, such as solar panels, sensors, other antennas (not related to producing the beams discussed above), etc.
FIG. 6 shows a seven-beam layout produced by the antenna system shown in FIG. 5. The center beam (Beam 1) is radiated by Antenna 1 having only one feed. Beams 2, 3, and 4 are produced by Antenna 2 having three feeds. Beams 5, 6, and 7 are generated by Antenna 3 using its three feeds.
FIG. 7 shows the feed apertures projected onto a common fictitious plane defined as a union of the three actual feed planes. There is a one-to-one correspondence between each beam in FIG. 6 and the feed represented in FIG. 7. In particular, central beam 1 is produced by the feed 14 of antenna 1 (B-1, A-1), beams 2, 3 and 4 may correspond to the 3 feeds (B-2, A-2; B-3, A-2; and B-4, A-2) of the cluster 16 in antenna 2, and beams 5, 6 and 7 may correspond to the 3 feeds (B-5, A-3; B-6, A-3; and B-7, A-3) of the cluster 22 in antenna 3. As shown in FIG. 7, feeds belonging to the same antenna do not interfere with each other, but the superposed feeds of different antennas overlap.
A four-aperture design including four antennas operates on the same principle as the three-aperture design discussed above, except that the alternate feeds are slightly further apart. It is used when it becomes necessary to achieve better efficiency, lower sidelobes, and larger separation between beams requiring co-polar isolation in the context of a frequency reuse scheme.
However, improvements achievable by the multi-aperture solution come at a significant cost because three or four antennas are used instead of one. Multiple antennas require large physical space, which may not be available on the spacecraft body, need separate support structures, multiply production, testing, alignment times, etc.
Therefore, it would be desirable to create a single-aperture antenna with a single main reflector and multiple feeds to produce closely spaced or overlapping beams corresponding to the feeds of the antenna system.